1. Field of the Invention
This invention relates to new photopolymerizable elements which can be used for preparing novel, dot-etchable image-containing masks that are useful for copy preparation and related purposes. Masks are used in imagewise exposing other photosensitive elements with actinic radiation.
2. Description of the Prior Art
In the photomechanical trades, a mask containing an image that is opaque to actinic radiation is used in preparing a printing plate of some kind; whether letterpress or lithographic, the method of production is much the same. A metal (or in some cases, plastic) plate is coated with a photosensitive resist-forming material, and is exposed to actinic light through the mask. After exposure, the plate is treated with a solvent that removes the unexposed material, leaving a resist image which protects the resist-covered areas of the metal plate from the acid used in etching the plate (in the case of letterpress) or from the various hydrophilic coatings used in lithography.
For this purpose, the image in the mask must be of the highest possible contrast, e.g., completely opaque black in the image areas, and completely transparent (free from fog) in the unexposed areas. For such masks, a film known as "litho" film (or various proprietary names containing "lith") is used. Such films are usually silver halide emulsions which produce extremely sharp images of high density and contrast; they are used in the graphic arts for making line and screen images, and in some phototypesetting systems. The characteristic curve of a lith film has a steeply rising straight-line portion, and a very short toe.
When a litho film is exposed through a halftone screen and developed, it contains an image comprised of dots. The dots correspond to the areas of the film under the transparent areas of the halftone screen and are comprised of exposed and developed material. In silver halide litho films, these dots may be reduced in size by "dot-etching". Dot-etching is reducing the size of, or "etching" the halftone dots, thereby changing the tone values of the image. In silver halide films, this is done chemically by treating the films with a silver "solvent". This method is used in lithography when tone values or color strength must be changed during the photographic steps rather than on the printing plate. Its importance is further illustrated in the processes for correcting color work.
When process color work is a part of the daily business of a lithograhic shop, a certain amount of correction is often necessary to accurately reproduce the colors in the original. A typical procedure requires the production of a set of three or four color separation negatives, and from these negatives, a set of halftone positives is made. Both of these operations are performed by a color cameraman, and at a later stage, the halftone positives are given to the dot etch artist for correction.
The dot etcher chemically treats the halftone positive dots to reduce their size in selected areas. At some point, the dot etcher is satisfied that the work he has done will produce the necessary color correction in the reproduction. He then turns the corrected positive over to someone to make a contact halftone negative from it for platemaking. This step is often performed by the contact printer. At times, if a deep-etch plate is required, the contact printer will first make a contact halftone negative, and then, from this negative, make a contact halftone positive. The reason for this seeming duplication of effort is to get an image with a dot sufficiently optically dense for the platemaking exposure. This type of dot is known as a "hard" dot. The dot etching process reduces the optical density of silver halide dots and therefore necessitates the extra step to obtain hard dots.
An improved mask suitable for use as a litho film is needed which will produce hard dots and which does not involve the expense and red light handling requirements of silver halide films. Any such mask, however, needs to have the characteristics of dot-etchability so as to permit tone correction.
Dot-etching of photopolymerizable elements is believed to be new. Due to the differences in materials and mechanisms, the production of dot-etchable photopolymerizable elements, which are particularly suited for use as litho films, has apparently not even been attempted.
In accordance with this invention improved photopolymerizable elements are provided which comprise a support bearing a photopolymerizable layer which is the outermost layer of the element or is contiguous to a removable cover sheet or an overcoat layer which is at least partially soluble in or permeable to a solvent for the photopolymerizable layer, characterized in that the photopolymerizable layer has an optical density in the actinic region of at least 3.0, and is no more than 0.0006 inch (0.015 mm) thick.
The photopolymerizable layer will ordinarily have a cover sheet or overcoat layer which is transparent to actinic radiation to protect the element against oxygen inhibition as well as physical damage. The invention therefore provides a photopolymerizable element comprising in order, (1) a support, (2) a photopolymerizable layer having a thickness of no more than 0.0006 inch (0.015 mm) and an optical density of at least 3.0 in the actinic region, and (3) a removable cover sheet or overcoat layer which preferably is transparent to actinic radiation. By the term "overcoat layer" as used throughout is meant a coating in dry form over the photopolymerizable layer which is a least partially soluble in or permeable to a solvent for the photopolymerizable layer.
The invention also provides a process of image reproduction having the steps of (1) imagewise exposing to actinic radiation a photopolymerizable layer on a support, the layer containing actinic radiation absorbing material, (2) removing the unexposed areas of the layer (e.g., by washing out from the side of the layer which was exposed to actinic radiation), leaving an image on the support comprised of the exposed areas of the layer, and (3) reducing the size of exposed image areas by removing the edges of the areas on the side of the layer which was exposed to actinic radiation.
The photopolymerizable elements of the invention can replace the use of expensive silver halide materials for making photograhic masks. It has been found that the invention, as defined, provides a photopolymerizable element with a characteristic curve suitable for lithographic applications and which produces images which are tone correctable by dot etching without significant loss of image density. The elements of the invention are also characterized by rapid processability, suitability, for bright light handling, and production of halftone dot images that are sharper and easier to reduce in size than those of silver halide litho film with greatly reduced operator handling. These and other aspects of the improvement in the art provided by the invention are described hereinafter .